The present invention relates to a process for the deposition by liquid phase epitaxy of a monocrystalline ternary compound of formula A.sub.x B.sub.n-x C.sub.m in which A, B and C are elements of the periodic classification of elements, n and m are integers and x is between 0 and n.
It is more particularly applicable to compounds of types III-V or II-VI, which are compounds of formula A.sub.x B.sub.1-x C in which A, B and C are elements belonging to groups II, III, V and VI of the periodic classification of elements and x is between 0 and 1. Furthermore, A and B belong to the same group of the periodic classification and C belongs to a different group.
Most of the compounds of this type are of considerable interest due to their semiconducting properties.
Among the known processes for the deposition of crystalline layers of ternary compounds of this type, liquid phase epitaxy is widely used and consists of growing on a monocrystalline substrate a layer of solid material depositied from a supersaturated liquid phase. The composition of the solid deposited on the substrate depends on the composition of the supersaturated liquid phase and the temperature used. For preparing the epitaxy bath, the quantities of the different constituents are initially weighed to have the desired composition and then the mixture of constituents is raised to the temperature necessary for forming the supersaturated liquid phase. The thus obtained bath is then contacted with the substrate to be coated by operating either at constant temperature, or at a decreasing temperature in order to compensate the composition variations of the bath during the growth of the epitactic layer.
For the preparation of ternary compounds of formula InAs.sub.1-x Sb.sub.x with x between 0 and 1, a liquid phase epitaxial deposition process is also known, in which an epitaxy bath is prepared by weighing appropriate quantities of polycrystalline indium arsenide, indium and antimony and by then heating the thus obtained mixture at the desired temperature for dissolving the indium arsenide and for saturating the bath. This is followed by deposition, by introducing the substrate to be coated and the indium arsenide source into the bath and by establishing a temperature gradient between the source of the binary compound InAs raised to temperature T.sub.1 and the substrate to be coated raised to temperature T.sub.2, which is lower than T.sub.1 (J. Electrochem. Soc., vol. 118, no. 5, 1971, pp. 805-810). In this way it is ensured that the liquid phase of the bath is subsaturated compared with the source of the binary compound InAs and that it is supersaturated compared with the growth substrate.
The layer can therefore be grown at a constant temperature, because the liquid phase is supplied with the two constituents of the bath by dissolving the source of the binary compound InAs. However, throughout the operation, it is necessary to establish a temperature gradient between the source and the growth substrate.
These known processes have the disadvantages of the initial composition of the epitaxy bath having to be accurately determined, which involves a careful calculation and control of the mass of the different constituents of the bath, whilst the composition of the layer obtained is not homogeneous over its entire thickness, which imposes certain limitations regarding the thickness of the deposited layers.
Thus, the composition of the solid generally differs from that of the liquid phase in equilibrium therewith. Therefore the composition of the epitaxy bath varies during the deposition operation and consequently there are variations in the composition of the deposited solid. However, in the case of the preparation of the compound InAs.sub.1-x Sb.sub.x, in which there is an excess of the binary compound InAs, a slower variation of the bath composition is obtained and this makes it possible to obtain thicker layers with a reduced composition gradient. However, in the latter case, it is necessary to use a special apparatus for establishing a controlled temperature gradient in the epitaxy crucible, which makes the epitaxy process more complicated and less suitable for industrial utilization.